The present invention relates to the analysis of circuits generally and more particularly to the distortion analysis of analog and RF (Radio Frequency) circuits by calculating distortion contributions of individual circuit elements.
Circuit linearity is a critical specification in analog and RF designs. Intermodulation product and compression are two commonly adopted measures for distortion [6, 11]. As a result, a general methodology for analyzing effects of individual components on circuit distortion and for identifying the major sources of nonlinearity is of practical importance in complex designs. From the designer point of view, it is desirable to separate total distortion into contributions of different elements. While in general such a separation is not possible for most nonlinear effects because of cross terms between devices, insights on distortion dependency on nonlinearity in a single device can still be obtained by conducting some heuristic analysis. At low RF power levels, perturbation methods based on Volterra series can be applied to calculate nonlinear effects [10, 2, 5, 1, 9, 4, 14, 15]. This approach can provide relatively detailed information related to distortion caused by different nonlinear terms in different elements.
A drawback of the Volterra method is that it generally requires high order device derivatives, which usually are not provided by device models used in real design simulations. As a result, the application of Volterra series has typically been limited to simple models and simple circuits. To circumvent this difficulty, Li and Pileggi proposed a post-simulation post-processing approach based on linear-centric circuit models, where the nonlinear current of an element is computed by subtracting the current in a linearized device model from the total current calculated by full device model evaluation [8]. According to this approach, the terminal voltage used for device evaluations is taken from the steady state solution which includes nonlinear effects of the whole circuit. Therefore, the nonlinear current obtained contains cross terms implicitly and is not totally independent of other elements.
In general, current methods do not adequately model circuit distortion in terms of contributions from separate device elements. Thus, there is a need for improved methods and systems for circuit distortion analysis, especially with applications to analog and RF circuits.